1. Technical Field
The invention relates generally to output driver circuits and, more specifically, to output driver circuits with programmable impedances.
2. Background Art
In electrical systems, output drivers are used to drive input/output (I/O) devices or similar loads. Each output driver is set up with a certain voltage/impedance that matches the strength of the transmission line and I/O device being driven by that specific driver. Thus, I/O devices with a low drive strength would need an output driver with a high impedance, and high strength I/O devices require a low impedance driver. Since each output driver typically has only one impedance rating, an output driver driving a load other than the one it is designed for would result in too much or too little of the strength needed. In addition, output driver impedance variations as a result of supply voltage, temperature, and process variations may be as high as 100% of the desired impedance. Consequently, such a system would suffer in performance from factors such as slow downs of a high performance system and/or dissipation of dc power. The mismatch between the SRAM Output Driver and the characteristic line impedance of the system is very undesirable in high performance and small signal applications, such as cache to processor I/O interfaces. Furthermore, if a separate part was designed for the many different load strengths across different systems, the costs may become expensive.
One solution to overcome using several single impedance output drivers is to use one output driver with a variable resistor external to the output driver. Therefore, a user may change the external resistor of the driver to reflect the voltage/impedance needed to drive a load.
The article entitled, "Digitally Adjustable Resistors in CMOS for High-Performance Applications" by Gabara et al., which is hereby incorporated by reference, discloses an output driver circuit with a digital adjustable resistor. The output driver circuit is employed with CMOS circuitry. The driver has an impedance that matches the transmission line being driven that is formed from a transistor whose width is adjusted by digital means. The overall impedance of the driver circuit is obtained through the circuit's "count". Counters are used to "lock" in that final count. Unfortunately, depending on whether the counters were in the process of counting up, or counting down, the final count obtained will be a different value. Thus, the tolerance of the system varies, which will degrade a system's performance. Furthermore, there are no means to save power in the system.
Accordingly, a need has developed in the art for an output driver circuit that will not only provide a variable impedance in its circuitry, but will produce the same final count, and thus, the same final impedance when supplied with the same reference impedance, regardless of the starting count.